Short circuit protector for transistorized circuits



Feb. 4, 9 J. w. TULLY, JR

SHORT CIRCUIT PROTECTOR FOR TRANSISTORIZED CIRCUITS Filed Sept. 19. 1960 Fig.

INVENTOR.

JOHN w. TULLY, JR.

1410:: 8a 14am Fig.

United States Patent 3,120,635 SHORT CIRCUIT PROTECTOR FOR TRANSISTORIZED CIRCUITS John W. Tully, Jr., San Diego, Calif., assignor to The Ryan Aeronautical Co., San Diego, Calif. Filed Sept. 19, 1960, Ser. No. 57,018 5 Claims. (Cl. 32322) This invention relates to an amplifier, and more particularly to a short circuit protector for a transistorized circuit.

Amplifiers and/or power supplies are widely used in present day electronic apparatus, and with the increasing need for smaller and lighter apparatus, much of this equipment is now transistorized. Unfortunately, transistors have the disadvantage that they are easily ruined by excess current, such as may result from a short circuit in or across their loads.

It is, therefore, the principal object of my invention to provide an improved circuit for protecting transistorized circuits from being overloaded.

The attainment of this object and others will be realized from the following specification, taken in conjunction with the drawing, of which:

FIGURE 1 shows the operation of my invention; and

FIGURE 2 illustrates another embodiment thereof.

Broadly speaking, my invention senses when a transistor is approaching its overloaded state and introduces circuitry that relieves the transistor of the need for passing excess current that may ruin it.

FIGURE 1 shows a transistorized regulated power supply wherein a battery supplies a load 12 with electrical current or voltage that should be of a fixed value. The intermediate circuitry is designed to minimize fluctuations, or to use a technical term, regulates the output.

As may be seen from FIGURE 1, the circuitry comprises a control transistor 24 whose collector-emitter circuit is connected in series between battery 10 and load 12. The bias potential applied to the base electrode of control transistor 24 determines the resistance of the transistor, and thus the current flowing therethrough.

The normal operation of the regulating circuit may be understood by assuming that for one reason or another the potential at point 14 drops. This may be due to conditions at source 1%, within or external to load 12, or resulting from the wiring etc. When the potential drops, the operation of voltage divider 16 is such as to apply a predetermined percentage of the drop to the base electrode of sensing transistor 18. Circuit 20, comprising a resistance and a reference voltage element, provides a fixed potential (or bias) for the emitter electrode of sensing transistor 18. The lowered potential applied from voltage divider 16 to the base of sensing transistor 18 produces a positive-going biasing signal at point 22 and control transistor 24 responds by reducing its resistance. This permits a larger current to flow through the control transistor, thus permitting a higher potential to appear at point 14 to compensate for the initial potential drop at that point.

While the above description has concerned itself with a drop of potential at point 14, an increase thereat causes a similar, but opposite, response from the regulating circuit. In this Way the circuit regulates, or compensates, for variations at the load.

There are times, when due to malfunctions or for some other reason, load 12 is short circuited as indicated by the dotted line. The potential at point 14 then drops toward zero and the inherent operation of the regulating circuit causes more and more current to pass through control transistor 24 in a vain attempt to regulate. The

3,126,635 Patented Feb. 4, 1964 result is that the control transistor is usually destroyed by the progressively increasing current passing therethrough.

Since transistors used in circuits such as this are very expensive, or may not lend themselves to easy replacement, there have been many proposals to overcome the above situation. Most of the prior art proposals have involved modifications that were almost as expensive as the transistors they sought to protect. Others required so much room that they voided the advantages gained by transistorizing the circuitry.

My invention, which overcomes the above problem, is extremely economical, requires practically no room and can easily be added to existing equipment without extensive modification. It has the additional advantage that it does not alfect the normal operation of the circuit and does not change the output impedance thereof.

It will be recalled that in FIGURE 1, a short circuit causes the potential at points 14 and 23 to drop toward zero, and causes the potential at point 22 to rise. This is a progressive action that progressively increases the emitter base bias potential, thus progressively decreasing the resistance of control transistor 24 and permitting a progressively increasing current to flow therethrough. To overcome this situation, I connect between the base and emitter electrodes of control transistor 24, an element 26 that has a voltage breakdown characteristic, i.e., it becomes conductive at a predetermined potential across it. Now when the potential between rising point 22 and falling point 23 increases above a predetermined value, element 26 becomes conductive. Its action may be thought of as limiting the base emitter bias of control transistor 24. Thus the resistance of this transistor is limited, and the transistor conducts only the amount of current it can safely handle.

I have found that the load can be intentionally short circuited, completely and indefinitely, without harming the control transistor. Once the short circuit is removed, element 26 becomes non-conductive, and the circuit performs in its normal manner.

There are a number of elements that exhibit this voltage breakd Wn characteristic, the most convenient one of which is a diode. Diodes, depending on their construction, have a low forward resistance that decreases when a suitably poled potential reaches a predetermined value. This gives rise to a characteristic known as an ease of current flow direction.

FIGURE 2 shows another embodiment of my invention. Here control transistor 24 comprises a series of cascaded transistors, shown as being of the pnp type, for providing an improved regulating action. Element 26 is now a plurality of voltage breakdown elements connected in series, so that the overall string becomes conductive at the desired predetermined potential. These elements are shown for convenience as diodes.

It should be noted that the voltage breakdown elements are not necessarily connected across the output terminal, but are connected across the base to emitter portion of the transistor to be protected. Furthermore, these elements are connected so that their ease of current flow is in the same direction as the ease of current flow of the base emitter portion of the transistor.

It may thus be seen that my invention solves the problem in an economical manner, that requires very little room, and does not afiect the normal operation of the circuit. While it has been explained in terms of a regulated power supply, it may be used to protect any type of transistor, in any type of circuit regardless of the circuit configuration.

It is understood that minor variation from the form of the invention disclosed herein may be made without departure from the spirit and scope of the invention, and

that the specification and drawing are to be considered as merely illustrative rather than limiting.

I claim:

1. The combination comprising: a source of potential; a load; a control transistor having a base electrode, an emitter electrode and a collector electrode; means for biasing said base electrode; means for connecting the collector emitter circuit of said transistor in series between said source and said load; means for sensing the potential changes at said load, said means comprising a voltage divider connected across said load and a sensing transistor having a base electrode, an emitter electrode and a collector electrode; a connection between said base electrode of said sensing transistor and a point on said voltage divider; means for biasing said emitter electrode of said sensing transistor, said biasing means comprising a voltage divider connected across said load, said voltage divider comprising a resistance and a diode and a connection between said emitter electrode of said sensing transistor and the junction of said resistance and diode; means for causing said sensing transistor to vary the potential at said base electrode of said control transistor to compensate for the potential changes at said load, said potential varying means comprising a connection between said collector electrode of said sensing transistor and the base electrode of said control transistor; a network, consisting of a plurality of series connected diodes, connected between said base electrode and said emitter electrode of said control transistor, said series of diodes having its forward resistance in the same direction as the ease of current flow of said base emitter circuit of said control electrode whereby said series of diodes limits the base emitter bias of said control transistor, and so prevents said control transistor from passing so much current through its collector emitter circuit as to endanger it.

2. The combination comprising: a source of potential; a load; a control transistor having a base electrode, an emitter electrode, and a collector electrode; means for connecting the collector emitter circuit of said transistor in series between said source and said load whereby the resistance of said control transistor may control the potential at said load; means for sensing potential changes at said load, said means comprising a voltage divider connected across to load and a sensing transistor connected to a point of said voltage divider; means for causing said sensing transistor to vary the potential at said base electrode of said control transistor, said means comprising a connection between said sensing transistor and said base electrode of said control transistor whereby said control 3. The combination comprising: a source of potential; V

a load; a control transistor having a base electrode, an emitter electrode, and a collector electrode; means for connecting the collector emitter circuit of said control transistor in series between said source and said load whereby the resistance of said control transistor controls the potential at said load; means for sensing potential changes at said load, said means comprising a sensing transistor; means for causing said sensing transistor to vary the resistance of said control transistor to cause said control transistor to compensate for said potential changes; an element, having a voltage breakdown characteristic, connected between said base electrode and said emitter electrode of said control transistor, said element having its ease of current flow in the same direction as the ease of current flow of said base emitter circuit of said control transistor whereby said element limits the base emitter bias of said control transistor, and so prevents it from passing an excessive current.

4. The combination comprising: a source of potential; a load; a control transistor having its collector emitter circuit connected in series between said source and said load; means for sensing potential changes at said load; means for causing said sensing means to control the resistance of said control transistor; an element, having a voltage breakdown characteristic, connecting the base electrode and the emitter electrode of said control transistor, said element having its ease of current flow in the same direction as the ease of current flow of said base emitter circuit of said control transistor.

5. The combination comprising: a transistor having its collector emitter circuit coupled in series with a load; a uni-directional device connected between the base and emitter of said transistor, said device having its ease of current flow in the same direction as the ease of current flow of said transistor.

References Cited in the file of this patent UNITED STATES PATENTS 2,832,900 Ford Apr. 29, 1958 

1. THE COMBINATION COMPRISING: A SOURCE OF POTENTIAL; A LOAD; A CONTROL TRANSISTOR HAVING A BASE ELECTRODE, AN EMITTER ELECTRODE AND A COLLECTOR ELECTRODE; MEANS FOR BIASING SAID BASE ELECTRODE; MEANS FOR CONNECTING THE COLLECTOR EMITTER CIRCUIT OF SAID TRANSISTOR IN SERIES BETWEEN SAID SOURCE AND SAID LOAD; MEANS FOR SENSING THE POTENTIAL CHANGES AT SAID LOAD, SAID MEANS COMPRISING A VOLTAGE DIVIDER CONNECTED ACROSS SAID LOAD AND A SENSING TRANSISTOR HAVING A BASE ELECTRODE, AN EMITTER ELECTRODE AND A COLLECTOR ELECTRODE; A CONNECTION BETWEEN SAID BASE ELECTRODE OF SAID SENSING TRANSISTOR AND A POINT ON SAID VOLTAGE DIVIDER; MEANS FOR BIASING SAID EMITTER ELECTRODE OF SAID SENSING TRANSISTOR, SAID BIASING MEANS COMPRISING A VOLTAGE DIVIDER CONNECTED ACROSS SAID LOAD, SAID VOLTAGE DIVIDER COMPRISING A RESISTANCE AND A DIODE AND A CONNECTION BETWEEN SAID EMITTER ELECTRODE OF SAID SENSING TRANSISTOR AND THE JUNCTION OF SAID RESISTANCE AND DIODE; MEANS FOR CAUSING SAID SENSING TRANSISTOR TO VARY THE POTENTIAL AT SAID BASE ELECTRODE OF SAID CONTROL TRANSISTOR 